Dispersion powders are polymer powders obtainable by spray-drying the corresponding aqueous polymer dispersions. A distinction is made here between dispersion powders which are redispersible in water and those which do not have this property. Water-redispersible polymer powders are generally obtained by drying the corresponding aqueous polymer dispersions in the presence of a drying aid (generally a protective colloid) and of antiblocking agent. Because of the protective colloid content, irreversible adhesion of the polymer particles is firstly prevented in the course of the drying operation, since the polymer particles are encased by the water-soluble protective colloid particles. Secondly, this protective colloid matrix, which redissolves when the polymer powder is dispersed in water, has the effect that the polymer particles are again present in the aqueous redispersion with the particle size of the starting dispersion.
To minimize powder blocking, especially in the case of dispersion powders based on polymers having a glass transition temperature Tg of <25° C., and to improve the free flow and flowability, antiblocking agents (anticaking agents) are added to the dispersion powders in the course of production thereof.
The antiblocking agent (ABA) serves to ensure free flow and storage stability of the polymer powder obtained after spray drying (dispersion powder). Antiblocking agents are an essential constituent of the dispersion powder especially when blocking of the powder or caking on the dryer wall are to be expected because of a low glass transition temperature of the polymer or high hygroscopicity. The powdering of the particle surface with antiblocking agent, because of relatively low agglomerate formation, more particularly, also reduces the proportion of coarse particles, which has a very advantageous effect on the overall drying operation and on the properties of the powder (for example free flow and storage stability).
Dispersion powders of this kind are used in many kinds of applications, including in coating compositions and in adhesives for a wide variety of different substrates. One example is use as a binding powder for particulate natural materials (Vinnex® powders from Wacker Chemie AG). The use thereof in chemical products for construction is widespread, frequently in conjunction with mineral binders (Vinnapas® powders from Wacker Chemie AG). Examples thereof are construction adhesives, especially tile adhesives, renders and mortar compounds, paints, spackling compounds, leveling compounds, thermal insulation composite systems and jointing mortars. The advantage of the water-redispersible dispersion powders lies particularly in the option of using them in prefabricated, storable dry mixes, optionally together with mineral binders such as cement, and of making them ready for use only immediately before use by addition of water. Dry mixes of this kind can be more easily transported (without a water content) compared to systems in paste form and offer advantages in storage, for example insensitivity to frost and resistance to microbe infestation.
DE-A 2214410 recommends metering the antiblocking agent into the spray tower in dry form, simultaneously with but separately from the aqueous polymer dispersion. Antiblocking agents having a mean particle size of 0.01 to 0.5 μm are recommended, since the finer the antiblocking agents the smaller the amount used. DE 3101413 C2 recommends hydrophobic silica having a particle size of 10 to 50 nm as an antiblocking agent. The addition is effected during the spray-drying in a mixture with the drying gas. In EP 690278 A1, the antiblocking agent is introduced with the drying gas below the atomizer disk of a disk atomization dryer. In the process from EP 1000113 B1, a mixture of hydrophobic and hydrophilic antiblocking agent is blown in in dry form during the atomization of the polymer dispersion.
In practice, the antiblocking agents are frequently in agglomerated form. This may, for instance in the case of precipitated or fumed silicas, be caused by the production process or exist as a material property. In other cases, for example in the case of use of kaolin, the agglomerate form is deliberately produced in the course of production by spray-drying an aqueous suspension of the antiblocking agent. This gives antiblocking agents with good free-flow properties, which can be conveyed without any problem and do not show any tendency to blocking or bridge formation in storage silos, which leads to significant advantages in the running of the process. A further advantage can be the higher bulk density of the agglomerated antiblocking agents, which is advantageous especially in the logistics sector. Such agglomerates, for example of kaolin, generally have a particle size of 10 to 250 μm, while the primary particles are about 0.01 to 5 μm in size (mean particle or agglomerate diameter determined by means of laser granulometry or scanning electron microscopy, SEM). Antiblocking agents in agglomerated form are also produced by means of extrusion processes, for example by extrusion of a highly concentrated aqueous suspension of antiblocking agent particles and subsequent drying. In this case, the agglomerates have particle sizes (mean agglomerate diameters) generally of 5 mm up to 5 cm.
From a process point of view, as mentioned, the use of the antiblocking agent in agglomerated form is preferable. However, a disadvantage of this form is that the surface area of the agglomerated antiblocking agent is reduced compared to the primary particles. This is the case irrespective of whether the agglomerate form is an intrinsic property of the antiblocking agent or has been deliberately generated by the manufacturer. Therefore, fewer antiblocking agent particles are available for powdering of the polymer powders, which makes the antiblocking agent less efficient. One option would be to mill the antiblocking agent by means of a solids mill and then to introduce it into the drying process. A disadvantage here is that the flowability of the ground antiblocking agent is again poor, combined with the disadvantages already mentioned in the process.